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Monday, April 6, 2009

Europa Hard Landers and Penetrators

This blog entry continues looking at presentations from the Russian-sponsored conference last January on Europa landers. The last entry looked at a proposal for a highly capable lander. This entry looks at proposals for two smaller landers.

Concepts for planetary landers fall into three classes. The first is for soft landers like the Mars Phoenix or MER rover craft. Combinations of parachutes, rockets, and or airbags soften the landing force. As a result, highly sophisticated craft and instruments can be delivered to the surface.

The other two classes of landers only partially spell the energy of descent and as a result do what can be termed as a controlled crash with style. Hard landers simply hit the surface, bounce and eventually come to a stop. Penetrators look like small rockets and burrow, nose first, into the ground and use the friction of ground penetration to stop some distance beneath the surface. Both classes of landers require hardened instruments and system electronics, which limit their capabilities. There isn't room for complicated masts to hoist imagers or robot arms to gather carefully identified samples for analysis. Instead, one or two simple instruments are carried. The landed mission lasts until the small batteries are exhausted.

The conference had presentations proposing both types of landers. The hard 'stop and drop' lander was discussed by a team from the Jet Propulsion Laboratory, one of NASA's centers. The team presented a number of possible instruments, but focused on an accelerometer (presumably to study the surface hardness but possibly also to measure some seismic activity), a gas-chromatograph/mass spectrometer, and a camera. The goal would be to conduct measurements for a full Europan day (84 hours) plus an additional 12 hours for additional data relay to the orbiter. Approximately 14 to 24 Mbits of data would be returned, depending on the altitude of the orbiter.

The penetrator presentations was done by the UK Penetrator Consortium. Here a small penetrator, perhaps 60 cm in length, would carry around 2 kg of instruments. Possible instruments include a seismometer, mass spectrometer, soil/environment package, simple surfve and descent cameras. Penetrators are used in a number of terrestrial studies, especially when dropped from airplanes. They have been studied for planetary missions for decades. Except for the doomed Deep Space 2 probes, none that I remember have flown. The Japanese space agency came close to flying penetrators to the moon, but canceled the mission due to development problems.

The hard lander presentation showed that for the nominal 2020 launch date and current mass estimates for the Jupiter Europa orbiter (JEO), 260 - 320 kg spare mass margin. (Note: spacecraft have a nasty tendency to grow in weight as design progresses and right now the JEO exists only as preliminary computer files.) No weight estimate is given for the hard lander plus descent system; presumably it would fit within the mass margin. One chart suggests a lander mass of 100 kg plus 35 - 65 kg of propellant. The penetrator presentation showed a mass of 30 kg or less for the penetrator and descent system. This would potentially allow several penetrators to be carried.

Editorial Thoughts: A number of problems exist with hard landers and penetrators. First, they require miniaturized desent craft that kill the orbital speed and possibly some of the descent speed. In the case of the penetrators, the descent craft also must ensure that the penetrator is pointing down for its impact. Another issue is the tight space in within the landers or penetrators -- every system must be miniaturized and hardened against impact, which reduces capabilities (including battery life). It is also hard to build miniaturized sample devices to bring material into the craft for analysis. This is especially true for hard landers which must be capable of acquiring samples no matter which side ends up being in contact with the surface. Europa would also poise a special problem for small surface landers -- there's not much mass to provide shielding against the radiation that will be present. Here, penetrators would have the advantage because the surrounding ice would provide some shielding.

Neither presentation gave more than passing mention to the radiation environment, suggesting -- as with the soft lander discussed in the last blog entry -- that these are preliminary concepts. I have my doubts about whether either would actually fly. On paper, the landers seem reasonable. As we learned from the Deep Space 2 and Beagle 2 landers, however, building and testing craft that would actually survive is hard and requires substantial financial resources.

However, should either concept make it to flight, I have a slight preference for the penetrator. I believe that it would be the lighter of the solutions, perhaps allowing 2 -3 to be carried. I suspect that crashing into the surface of Europa could be fatal a high percentage of the time. Redundancy would be nice.

If a lander is designed for Europa, then in theory it could be modified to also fly on ESA's Jupiter Ganymede Orbiter (JGO). The primary difference that I see would be the need for a heftier descent system to account for the greater mass of Ganymede.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.